Definition
A galaxy is an astronomical structure composed of billions of stars, interstellar gas, cosmic dust, and dark matter, bound together by gravity. It forms a dynamic unit in the universe, distinct from galaxy clusters or superclusters, and often appears as a coherent luminous entity in the sky.
Structure
The structure of a galaxy can include several components: a central bulge, often spherical, a flat disk with spiral arms or extended features, a diffuse halo of old stars, and an invisible component of dark matter. Mass distribution and brightness vary depending on the galaxy’s morphology.
Function
Galaxies act as a framework for star formation, evolution, and disappearance. They enable material aggregation, regulation of gas flows, creation of planetary systems, and large-scale gravitational interaction. They organize the universe on a larger scale than individual stars.
Evolution
Galaxies evolve through merging, accretion, gravitational interaction, and internal transformation. Their shape and composition can change over time. Collisions between galaxies alter their structure, trigger star formation, or fuse their cores. Their evolution depends on gas content, surrounding forces, and cosmic time.
Boundaries
A galaxy’s boundaries are not clearly defined. They are determined by the decrease in star density, extent of gravitational influence, and spread of dark matter. Galaxies are not isolated: they constantly interact with their cosmic environment, including nearby galaxies, galactic filaments, and the cosmic background.